Effects Of Vanadium Substitution Research Articles

The Effects of Vanadium Substitution on One-dimensional Tunnel Structures of Cryptomelane: Combined TEM and DFT Study

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The effects of vanadium substitution on one-dimensional tunnel structures of cryptomelane: Combined TEM and DFT study

Rational design of energy storage materials requires mechanistic understanding which relies on atomic scale characterization of structure and defects. Here we utilize vanadium (V)-substituted one-dimensional cryptomelane as a model material to study structural features critical to electrochemical reversibility of the cathode in lithium-ion batteries. We found the substitution of manganese (Mn) atoms in the tunnel wall with V enhances structural uniformity in the parent KMn8O16 material by inhibiting formation of irregular (2 × 3 and 2 × 1) manganese oxide tunnel structures. Upon lithiation, both transmission electron microscopy (TEM) and density functional theory (DFT) reveal significant structural evolution, eventually resulting in phase separation and formation of highly reduced manganese oxide. During this process, Mn4+ accepts electrons donated by intercalated Li, while the V4+ substituent is not electrochemically reduced but acts to stabilize the tunnel framework structure. As a result, the V substituted material shows higher operating voltage and improved electrochemical reversibility. Our results highlight the benefits of V-substitution in cryptomelane, and the significance of integrating characterization with simulation for understanding and developing design principles for lithium ion battery materials.

Effect of V substitution on vortex pinning and superconducting properties of Bi-2212 superconductor

In this work, the effect of Vanadium (V) substitution in the Bi2Sr2Ca1−xVxCu2O8+y superconductor with x = 0.0, 0.05, 0.1, 0.2, 0.3, and 0.4 has been investigated. The samples were prepared by a conventional solid state reaction technique. The effects of vanadium substitution have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, and magnetic characterizations. XRD patterns indicate that the Bi-(2212) phase is the major one for pure sample, but with increasing of V content, the volume fraction of Bi-2212 is reduced and transformed to mainly low-Tc phase, namely Bi-2201. 0.05 V substitution causes a remarkable increase of superconducting critical temperature as 86.2 K, which is higher than pure sample by 2.3 K. Moreover, 0.05 V leads to improvement of intragranular JC value which is calculated as 2.53 × 104 A/cm2 at 10 K. For comparison, the enhancement in JC value is about 78 % more than those value obtained for pure sample. The whisker structure with a length of 0.25 mm in main matrix is also observed in the SEM micrographs of 0.4 V substitution.

Enhancement of superconducting properties of Bi-2212 ceramics by vanadium–sodium co-doping

Bi2Sr2−xVxCa1Cu1.75Na0.25Oy superconductors with x = 0, 0.05, 0.1 and 0.25 were prepared by the classic solid-state reaction method. The effects of vanadium substitution have been investigated by means of X-ray powder diffraction, scanning electron microscopy, dc electrical resistivity, and magnetic hysteresis loop measurements. Results show that the superconducting properties of Bi-2212 phase can be significantly improved with x = 0.05 vanadium substitution. SEM micrographs show the presence of plate-like crystals in all samples, implying the composition of 2212 phases. In addition, Jc values of the samples were calculated from the hysteresis loop measurement by using the Bean’s model showing that Jc significantly increases with x = 0.05 vanadium substitution.

Effects of vanadium substitution on the electrical performance of amorphous SrBi2Ta2O9 thin-film capacitors

The effects of vanadium substitution on the dielectric properties of amorphous SrBi 2 Ta 2 O 9 (SBT) thin films have been investigated. Vanadium substitution at the Ta site exists in the V 3+ valence state and acts as an acceptor, reducing the number of intrinsic oxygen vacancies and the leakage current of the amorphous SBT thin films. Furthermore, the dielectric properties are also improved. The leakage current values of the 92 and 31 nm thick SBTV thin-film capacitors were 8.8 nA cm −2 and 0.62 μA cm −2 at 1 V, respectively.

Effects of vanadium substitution on the cycling performance of olivine cathode materials

V-substituted LiFePO4 (LiFe1−xVxPO4, 0 ≤ x ≤ 0.10) powders are prepared via a solution method. The compositions, crystalline structure, morphology of the prepared powders are systemically investigated with inductive couple plasma-option emission spectrometry (ICP-OES), synchrotron radiation X-ray diffraction (s-XRD), X-ray Absorption Spectroscopy (XAS) and field emission-transmission electron microscopy (FE-TEM). Results of s-XRD and Rietveld analysis reveal that olivine phase is observed exclusively for LiFe1−xVxPO4 (x ≤ 0.05) samples, whereas a minor amount of monoclinic Li3V2(PO4)3 is detected in the prepared LiFe0.93V0.07PO4 and LiFe0.90V0.10PO4 samples. It is also found that the lattice parameters of olivine structure and average Li–O and M–O (M = Fe or V) bond lengths slightly increase and decrease with the amount of V substitution, recommending an enhanced lithium diffusivity in the structure. Results of XAS study suggest that V ions occupy octahedral sites (4c) of Fe in LiFePO4 structure with valence of 3+, showing good agreement with shortened M–O bonds determined in Rietveld analysis. As a result of V substitution, LiFe1−xVxPO4, (0 ≤ x ≤ 0.10) cathodes exhibit better electrochemical performance, such as discharge capacity, rate capability, and cycling performance at both room and elevated temperatures.

Synthesis, characterization and electrochemical performance of Li2FeSiO4/C cathode materials doped by vanadium at Fe/Si sites for lithium ion batteries

Li2FeSiO4/C composites doped by vanadium at Fe/Si sites have been investigated as cathode materials for lithium ion batteries. Effects of vanadium substitution at different sites on the structure of Li2FeSiO4/C are examined by X-ray diffraction, X-photoelectron spectroscopy and scanning electron microscopy. XPS results show that the oxidation state of vanadium doped at Fe sites is +3, whereas is +5 when doped at Si sites. Electrochemical measurements show that the Li2FeSi0.9V0.1O4/C sample exhibits the best electrochemical performance with initial discharge capacity of 159mAhg−1 and excellent cyclability with capacity of 145mAhg−1 at 30th cycle, which can be ascribed to larger cell volume and higher lithium ion diffusion coefficient, however, the initial discharge of the Li2Fe0.9V0.1SiO4/C sample is only 90% of the undoped Li2FeSiO4, which can be attributed to the loss of Fe content.

Structure and transport properties of vanadium substituted cerium niobate

The effects of vanadium substitution on the structure and transport properties of CeNbO 4 + δ were investigated. Initial studies were via room temperature X-ray powder diffraction of the CeNb 1 − x V x O 4 series ( x = 0, 0.2, 0.4, 0.6, 0.8 and 1) identifying three distinct phases; monoclinic fergusonite ( I2/ a), tetragonal wakefieldite ( I4 1/ amd) and tetragonal scheelite ( I4 1/ a). The scheelite phase occurred at a stoichiometry of CeNb 0.6V 0.4O 4 and is of interest due to its open fluorite-like structure believed to be beneficial for oxide ion conduction. This phase was therefore the main focus of the investigation and was examined using high temperature X-ray diffraction, thermogravimetric analysis and dilatometry, all of which identified a thermally and redox stable phase in contrast to the parent material. The room temperature structure was confirmed using neutron powder diffraction and the transport characteristics of the CeNb 1 − x V x O 4 series probed by AC impedance spectroscopy, highlighting an increase in conductivity with increasing vanadium substitution.

Effects of Vanadium Substitution on the Structure and Photocatalytic Behavior of ETS-10

A combination of experimental and computational methods has been used to investigate the effects of vanadium doping in ETS-10. Near edge X-ray absorption fine structure (NEXAFS) spectra reveal octahedrally coordinated VIV and VV species within V-doped ETS-10 materials, confirming substitution for TiIV sites only. Computational models, using hybrid density functional theory/molecular mechanics (DFT/MM) methods, have been developed that contain varying concentrations of VIV and VV within the O−M−O (M = Ti, V) chain. Geometry optimizations indicate that VV substitution leads to larger changes in the local chain geometry than VIV substitution. Substitution energetics for VIV and VV in different sites have been calculated to determine preferred locations of the two species, suggesting that long chains of VV are not stable and demonstrating the need for both VV and VIV within V-substituted materials. Wavefunctions for systems with an electron added or removed are used to identify electron and hole trapping site...

Magnetic behaviour and site occupation of YFe12-x V x (x=2,3,4)

The effects of vanadium substitution on the iron magnetism in YFe12-xVx compounds (x∼2–4) have been studied by57Fe Mossbauer spectroscopy. The preference of vanadium atoms for the 8i site of these ThMn12-structure type compounds is confirmed. The rapid collapse of the iron sublattice magnetization with increasing vanadium concentration is analysed in terms of the ‘magnetic valence’ model.

Magnetic and structural studies in Sm-Fe-Ti magnets

The effects of vanadium substitution on the magnetic properties of Sm-Fe-Ti-V melt-spun ribbons and the magnetic properties and microstructure of sintered Sm-Fe-Ti based magnets are reported. The highest coercivity, Hc =10.65 kOe, was obtained in a heat-treated melt-spun Sm8 Fe75.5 Ga0.5 Ti8 V8 alloy. This is the highest value of the coercivities reported in alloys with the ThMn12 -type structure. Bulk magnets were prepared by the usual powder metallurgy technique. However, their highest coercivity (∼2 kOe) was much less than that of the corresponding ribbons. The low coercivity of sintered magnets has been attributed to their nonuniform microstructure.

Substitutional studies on the anisotropic, semiconducting, molybdenum bronze, Li 0.33MoO 3

Pure Li 0.33MoO 3 and vanadium-doped Li 0.33Mo 1− x V x O 3 crystals were grown by a temperature gradient flux technique. Directional electrical resistivity measurements have been made on both the pure and doped crystals. The electrical properties of Li 0.33MoO 3 are highly anisotropic with resistivity values at room temperature of 1.8 × 10 −1 and 6.0 × 10 2 Ω cm along the c and b∗ directions, respectively. Li 0.33 MoO 3 shows complex semiconducting behavior and an anomalously low paramagnetic signal. The effects of vanadium substitution on the structure and transport properties of Li 0.33MoO 3 are dramatic. Attempts are made to correlate the observed electrical and magnetic behavior with the structure of Li 0.33MoO 3.